HarryM - 11:30 am on Dec 31, 2007 (gmt 0)

Although supplementals may not be the complete answer to this issue, I believe they pay some part in it.

Early in 2007 many sites experienced up to 50% of their pages going supplemental, and they just dropped out of the standard SERPS. In my case supplementals could only be found if they were deliberately searched for, e.g., by putting a specific search term in quotes.

Later in 2007, because of the furore this caused, Google began selectively to merge results from the two indexes and the supplementals reappeared, often on the first page. But now Google treats both indexes as one, and supplemental pages that used to be at the top of SERPS in 2006 are now back to their original positions. Only Google knows what proportion of pages is in the supplemental index, but this must be a very large number of pages and it has to influence what is being seen today.